The principal of electrostatic attraction has been used for many years to enhance the removal of contaminants from air streams. There are three primary categories of air electrostatic cleaners: electrostatic precipitators, passive electrostatic filters and active field polarized media air cleaners, which are sometimes known under different terms.
Electrostatic precipitators charge particles and then capture them on oppositely charged and/or grounded collection plates.
A passive electrostatic filter (also known as an electret) employs a media (or combination of different media) that through some combination of treatment and/or inherent properties has an electrostatic charge. Particles entering the filter media that have an electrostatic charge are attracted to the charged media filter materials that have the opposite electrostatic charge.
An active field polarized media air cleaner uses an electrostatic field created by a voltage differential between two electrodes. A dielectric filter media is placed in the electrostatic field between the two electrodes. The electrostatic field polarizes both the media fibers and the particles that enter, thereby increasing the efficiency of the media and the air cleaner. A dielectric material is an electrical insulator or a substance that is highly resistant to electric current that can also store electrical energy. A dielectric material tends to concentrate an applied electric field within itself and is thus an efficient supporter of electrostatic fields.
A further electrostatic air filter design is disclosed in Canadian Patent No. 1,272,453, in which a disposable rectangular cartridge is connected to a high voltage power supply. The cartridge consists of a conductive inner center screen, which is sandwiched between two layers of a dielectric fibrous material (either plastic or glass). The two dielectric layers are, in turn, further sandwiched between two outer screens of conductive material. The conductive inner center screen is raised to a high voltage, thereby creating an electrostatic field between the inner center screen and the two conductive outer screens that are kept at an opposite or ground potential. The high voltage electrostatic field polarizes the fibers of the two dielectric layers.
Pleated filters are also well known. A pleated filter is formed from a sheet of filter media folded into a series of pleats. One type of pleated filter, known as a mini-pleat filter, has smaller more closely spaced pleats. The peaks between adjacent pleats of a mini-pleat filter are spaced no more than 20 mm apart and typically range from 5.0 mm to 7.0 mm apart.
Mini-pleat air filters typically utilize ⅞ to 1¼ inch deep pleats with very narrow air spaces (⅛ inch) between, making it possible to pack more filter paper into a standard frame than can be done with traditional deep, corrugated pleats. The abutting folds of a mini-pleat filter are separated and held in place by glue beads, threads, ribbons, tapes, strips of medium, or a continuous piece of glass, foam or plastic spaced within the width of the medium. Mini-pleat filters contain almost twice as much filter paper as deeply pleated filters or corrugated separator filters of equal frame size.
U.S. Pat. No. 2,908,348 to Rivers shows the use of conductive stripes applied to a pleated media for generating electrostatic fields. The stripes serve to create an electrostatic field within the pleated filter media.
U.S. Pat. No. 6,497,754 to Joannou shows a pleated filter with conductive strings attached to the top edges and bottom edges (peaks) of the pleated folds. Applying a high voltage potential between the top and bottom peaks of the pleated folds generates an electrostatic field within the pleated filter material.